Very often fluids such as oil, natural gas and the like are transported to or from offshore installations or from one coastal line to another through long continuous pipeline systems laid out on the seabed. The pipelines therefore naturally have to be able to sustain the very special environmental conditions and extreme structural demands arising from the surroundings such as external pressure up to 20 bars or more, a very corrosion aggressive environment combined with high demands on the tightness on the pipes and all joints, extreme loadings during installation, resistance to wear, etc.
Different types of pipelines are used for such offshore applications including simple single un-insulated pipes, pipe-in-pipe systems with or without insulation material between the pipes and pre-insulated composite pipelines. The type of pipeline system depends for one thing on the fluid to be transported. In applications of transporting oil, the oil is very often mixed with some gasses and water resulting in methane hydrates forming in the mixture. If the pipelines are not sufficiently thermally insulated, the cooled methane hydrates solidify on the pipe walls eventually clogging the pipeline. In order to avoid this, additives such as methanol and glycols are added to the oil/gas mixture which then, however, is to be boiled off at the receiving station and returned for reuse. Thermally insulated pipelines are also often necessary as the viscosity of some crude oils is too high to be pumped if the temperature of the oil becomes too low.
Alternatively, this issue can be avoided by the application of thermally insulated pipelines comprising an inner carrier pipe in which the fluid runs, an exterior casing or pipe and insulation material filling up the space there in between. Such an insulated pipe is disclosed in GB 2,407,857, which comprises a first and a second tubular and insulation material there between. Due to its low costs, the insulation material used is mineral wool which on its outer surface facing the second tubular is covered with a member such as a polymer. The mineral wool and the resilient member are helically arranged in the space defined by the first and second tubular. When the insulation pipe is rolled onto a reel, the mineral wool will open to overcome tensile stress caused by the bending of the insulation pipe. In order to center the first and second tubular relative to each other, this insulated pipe further comprises a number of spacers placed between the first and second tubular. In addition to the purpose of functioning as centralizers the spacers also serve to transfer the loads between the two tubular. The resilient mineral wool is therefore not damaged when this insulated pipe is wind and rolled onto reels and of the reels again. Thus the spacers are able to transfer the compressive stress. This however requires that the second tubular is rigid such that the bending moments exerted on this second tubular can be transferred to the first tubular. Last but not least the second tubular has to be rigid in order to withstand the inevitable pressure exerted on the pipe assembly when it lies on the seabed. A lack of sufficient rigidness of the second tubular would therefore lead to a bending of the second tubular casing along the distance between the spacers. This would eventually decrease the insulation properties of this insulated pipe, as the distance or space between the first tubular and the second tubular would vary along the whole pipeline. To be able to transfer the loads between the very rigid first and second tubular the distance between the spacers will have to be short. Since every spacer constitutes a potential thermal bridge, this would decrease the insulation properties of this insulation pipe. Having a rigid first and second tubular is expensive. These costs are further raised due to the cumbersome, labor intensive and time consuming application of spacers and the mineral wool during the manufacture of this insulated pipe. A sufficient rigidness of both tubular can be obtained by using metal. Since the seawater would be highly corrosive to the second tubular of metal, some sort of treatment of the outer surface of this tubular is required. This would add further to the costs of this insulation pipe. From a technical, insulating, and costwise viewpoint the insulated pipe disclosed in GB 2,407,857 is therefore not desirable.
Another alternative is disclosed in CH 633 092. Here the inner pipe is surrounded by a flexible corrugated pipe of synthetic material, which again is surrounded by a layer of foam material. Between the layer of foam material and the corrugated pipe a band of polyester is applied in order to prevent the foam material from migrating into the open ‘valleys’ of the corrugated pipe. The insulated pipe disclosed in CH 633 092 cannot be suitably applied on a seabed as the environment here is destructive for the outer foam material. In addition there is a great risk that the corrugated pipe will collapse due to the pressure present here. Further this insulated pipe will be difficult to roll on to a reel, as the corrugated pipe cannot transfer the bending moments exerted on the foam material to the inner pipe. This problem arises as the corrugated pipe is not bonded to either the inner pipe or the foam material. Hence there is a risk that the corrugated pipe will slide on the inner pipe and potentially collapse due to the bending moment exerted.
An alternative to this solution is to use closed-cell insulation foam between an inner carrier pipe and a lighter casing. The thermal insulation may also consist of one or more layers of solid or partly foamed thermoset or thermoplastic polymers. In order to transfer all thermal and structural loads from the inner carrying pipe to the exterior and vice versa an absolute and complete adhesion and bonding of the insulation material to both the inner pipe and the exterior casing is important. However, this bonding together with the necessary relatively high stiffness of the insulation layer due to the water pressure results in a pipeline which is relatively stiff and inflexible. Such a pre-insulated pipeline is thus not capable of being bent without severe damage to the exterior casing. Therefore, the pipelines are not capable of being wind and rolled onto reels, but will have to be manufactured in a number of straight and thus shorter sections to be assembled later on the spot. In offshore applications this implies a much slower installation process on board of an installation ship barge where the sections are welded together under difficult and unfavorable weather conditions. This unavoidably results in a far more expensive installation process, but also in higher risks of lower quality welds and joints than otherwise obtainable under well-controlled conditions in a production facility or compared to the installation processes of pipelines initially winded onto huge reels or spools.